Last month Japan marked the second anniversary of the March 11 earthquake that sent a huge wall of water into its north-eastern coast, killing around 19,000 people and triggering a nuclear calamity.

The tsunami created the biggest single dumping of rubbish, sweeping some five million tonnes of shattered buildings, cars, household goods and other rubble into the sea.

An estimated three-and-a-half million tonnes sank immediately, leaving some 1.5 million tonnes of plastic, timber, fishing nets, shipping containers, industrial scrap and innumerable other objects to float deeper into the ocean.